Exhaust flow in a turbocharger system may be partially directed to an exhaust driven turbine to drive a compressor that delivers air into engine cylinders, while the remaining portion of the exhaust gas may be flowed via a bypass passage around the turbine to an exhaust catalyst placed downstream of the turbine. The bypass passage may include a wastegate valve that may be adjusted into an open position, thereby allowing exhaust gases to bypass the turbine and flow to the catalyst. The wastegate valve may be adjusted into a closed position that at least partially blocks exhaust flow in the bypass passage and thus most of the exhaust gas delivered from the engine is directed to the turbine. During an engine cold start, exhaust gas may be at least partially routed through the bypass passage and wastegate in order to direct exhaust flow to a front face of the catalyst, thereby enabling catalyst light off to be reached more quickly.
Other engine systems may include a split exhaust manifold wherein at least some exhaust from a first set of cylinders is directed to a first scroll of a turbocharger turbine via a first exhaust manifold and at least some exhaust from a second set of cylinders is directed to a second scroll of a turbocharger turbine via a second exhaust manifold. In these systems, the at least some exhaust from the first and second set of cylinders may be diverted away from the first and second scrolls and instead routed to a bypass passage coupled to both the first and second exhaust manifolds, the bypass passage having a wastegate that may be adjusted between a closed and open position to direct exhaust flow to a catalyst positioned downstream of the wastegate and the turbocharger turbine.
One example design of a wastegate valve in an exhaust passage of a turbocharger is disclosed by Grabowska in U.S. patent application 2015/0345375. Therein, a wastegate valve assembly having flow formations is provided to direct exhaust gas in a primary flow direction while reducing exhaust losses in secondary flow directions. Specifically, the wastegate includes flow formations on a valve body, supported on a valve arm that is pivotally supported on a turbine housing. Example flow formations on the valve body include a concave shaped disc, shallow ribs and an extended semi-circular surface formed on the valve body to direct exhaust flow in the primary direction.
The inventors herein have recognized potential issues with the example approach disclosed above. For example, in the valve body configured with the concave shape disc or shallow ribs, exhaust flow may fan out in multiple directions, impinging on turbocharger walls and creating turbulent flow conditions. As a result, exhaust energy may be transmitted to the turbocharger walls leading to reduced flow efficiency and energy losses. Also, since the flow fans out in multiple directions before flowing downstream, less exhaust heat may reach the catalyst and thus delayed catalyst lightoff may occur.
The inventors herein have developed a wastegate design to at least partly address the above issues. In one example design, a wastegate may be provided comprising: a valve plate having an interior with a multiplane curved surface and a first mating feature centered along the curved surface, the curved surface forming a raised edge and a side opening on opposite sides of the valve plate; and a passage bifurcated by a central wall, an end of the central wall including a second mating feature adapted to have face-sharing contact with the first mating feature.
In this way, the design of the wastegate may be used to improve flow efficiency and reduce energy losses in the turbocharger while improving catalyst lightoff. For example, the multiplane curved surface on the valve plate may act in conjunction with a constricted section in the passage to guide exhaust flow and increase flow velocity downstream of the wastegate. In another example, the central wall may divide the wastegate passage into a first and a second side, where the first side receives exhaust flow from a first scroll coupled to a first group of cylinders and a turbine, and the second side receives exhaust flow from a second scroll coupled to a second group of cylinders and the turbine. In this way, the wastegate design may confer several advantages. By directing exhaust flow downstream instead of fanning out in multiple directions, the wastegate may reduce exhaust energy losses to turbocharger walls. Further, the constricted section in the passage may allow the exhaust flow to speed up before exiting the wastegate. By providing a bifurcated passage coupled to the first and second scrolls of a turbocharger, exhaust flow from the two particular groups of cylinders may not communicate with one another.
For example, the two groups of cylinders may be established such that in the firing order of the engine, the exhaust from subsequent cylinders alternates between the first scroll and the second scroll. Such a configuration can allow a four cylinder engine to use exhaust valve lift durations that exceed 180 degrees without having the high exhaust manifold pressure of one cylinder at the beginning of the exhaust event pushing exhaust gas into the previous cylinder at the end of its exhaust event. Exhaust valve lift durations greater than 180 degrees are desirable to improve pumping efficiency of the engine. Additionally, using a manifold and turbocharger with separated passages allows for smaller volume between the cylinder exhaust valves and the turbine which increases the conversion of blowdown exhaust energy into turbine work. This can improve the fuel economy and transient performance of the vehicle. Further, by including a first mating feature on the valve plate and a second mating feature on the central wall of the passage, when the valve plate is closed against the passage, the first and second mating features may seal against one another, thereby reducing exhaust flow communication between the two sides of the passage (and thus the two scrolls of the turbine). In this way, the wastegate valve may reduce exhaust energy losses to improve catalyst lightoff conditions while reducing fuel emissions and increasing performance of a twin scroll turbocharger.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.